Today several different types of plate heat exchangers exist, which are employed in various applications depending on their type. One certain type of plate heat exchanger is assembled by bolting a top head, a bottom head and four side panels to a set of corner girders to form a box-like enclosure around a stack of heat transfer plates. This certain type of plate heat exchanger is often referred to as a block-type heat exchanger. One example of a commercially available block-type heat exchanger is the heat exchanger offered by Alfa Laval AB under the product name Compabloc.
A block-type heat exchanger typically has fluid inlets and fluid outlets arranged on the side panels while baffles are attached to the stack of heat transfer plates for directing a fluid back and forth through channels formed between heat transfer plates in the stack of heat transfer plates.
Since the stack of heat transfer plates is surrounded by the top head, the bottom head and the four side panels, the heat exchanger may withstand high pressure levels in comparison with many other types of plate heat exchangers. Still, the block-type heat exchanger is compact, it has good heat transfer properties and may withstand hard usage without breaking.
The stack of heat transfer plates is sometimes referred to as a plate pack and has a special, block-like design that is characteristic for block-type heat exchangers. The stack of heat transfer plates is often all-welded and no gaskets are needed between heat transfer plates for proper sealing of flow channels that are formed between the plates. This makes a block-type heat exchanger suitable for operation with a wide range of aggressive fluids, at high temperatures and at high pressures.
During maintenance of the block-type heat exchanger, the stack of heat transfer plates may be accessed and cleaned by removing e.g. two side panels and flushing the stack of heat transfer plates with a detergent. It is also possible to replace the stack of heat transfer plates with a new stack, which may be identical or different from the previous stack as long as it is capable of being properly arranged within the heat exchanger.
Generally, the block-type heat exchanger is suitable not only as a conventional heat exchanger but also as a condenser or reboiler. In the two latter cases the heat exchanger may comprise additional inlets/outlets for a condensate, which may eliminate the need for a special separator unit.
In some situations, a block-type heat exchanger comprising free-flow channels for one of the fluids, i.e. channels inside which there is no contact between the heat transfer plates defining the channels, is required. For example, in applications with particularly high demands on hygiene, such as pharmaceutical applications, a plate heat exchanger with free-flow channels is often required. This is because the lack of contact points between the heat transfer plates renders the cleaning of the associated free-flow channel much easier. Further, a free-flow channel enables an ocular inspection of the complete channel to assure that it is clean. As another example, in connection with high fouling applications, free-flow channels enable handling of fluids containing fibers and solids with a relatively low risk of plugging since there are no obstacles to the flow inside the free-flow channels. Also here, the easy cleaning of the free-flow channels is of course an advantage.
The existing heat exchangers comprising free-flow channels functions very well for applications where the pressure inside the free-flow channels is higher than the pressure outside the heat exchanger. However, for applications where the pressure outside the heat exchanger is higher than the pressure inside the free-flow channels, there is a risk of deformation, more particularly compression, of at least the outermost free-flow channel. Naturally, this could negatively effect the performance of the plate heat exchanger.